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Author: Andrew Bell

Andrew C. Bell, Ph.D., serves as curator of woody plants and is responsible for managing the development of the Garden's tree and shrub collections. Dr. Bell's work includes evaluating and promoting woody plants for sustainable landscapes and studying the effects of climate change on Chicago's urban forest.

It is our responsibility as citizens, but especially as practitioners in the fields of horticulture and botanical sciences, to be good stewards of the land and ensure that what we are growing in our backyards and at the Chicago Botanic Garden will not contribute to problems in the future. That is why the Garden recently replaced the callery pears at the entrance to the Visitor Center.

In addition to the callery pear, the Garden also removed winged euonymus (Euonymus alatus) and tamarisk (Tamarix spp.).

Callery pear and euonymus plantings at the Visitor Center entrance in autumn of 2010.

One threat to our natural world is invasive plants. While many of these plants may on the surface appear to be attractive additions to the landscape, they can force out native species. Short of complete destruction of a natural area, I cannot think of anything more unsightly than a natural area that has been completely consumed by an invasive species to the point that it is no longer recognizable and holds very little biodiversity.

The list of invasive plants and potentially invasive plants is not set in stone; it is an evolving list and one that will continue to change as our climate changes. The callery pear (Pyrus calleryana) was not on the invasive species lists for our region a decade ago, but today it is in Illinois, along with Alabama, Georgia, South Carolina, and Tennessee. Most are familiar with the cultivar ‘Bradford’, but there are several others, including ‘Aristocrat’, ‘Autumn Blaze’, and ‘Cleveland Select’.

Garden staff plant non-invasive American yellowwood (Cladrastis kentukea) at the entrance to the Visitor Center.

This spring we replaced all of the Pyrus calleryana ‘Autumn Blaze’ at the front entrance of the Visitor Center with the non-invasive American yellowwood (Cladrastis kentukea). As a result, the front entrance is noticeably more open in appearance, and the low canopy and refreshing shade that existed in years past is now gone; it will return as the plantings grow.

Although both of these species are medium-sized deciduous trees with white flowers, the yellowwood has different ornamental characteristics. The callery pear flowers in the early spring, while the yellowwood flowers in late spring to early summer. The callery pear has unrivaled fall color in shades of red, orange, and yellow, while the yellowwood is one of the best for yellow fall color.

Why is the callery pear called an invasive?

This pear has abundant seeds that can be carried by birds to natural areas. Plants can then become established, thus displacing native species. As land stewards, the Garden is very mindful of prohibiting and eliminating any plants known to be invasive in our region.

The new plantings at the Visitor Center entrance will take time to grow, but will someday provide the shade and fall color of their predecessors.

I have no doubt that some of these recently removed invasive plants were favorites among Garden visitors, our staff included. But sometimes what we like isn’t always good for us, or good for the environment.

New seasonal plantings replace the previous shrubs in the entry beds.

I encourage you to review and continue to consult invasive species lists, including on the invasive.org website, and do your part by removing invasive species from your garden and resist purchasing and adding more. There are so many benign, beautiful options available to gardeners, and the Garden, along with numerous other organizations, has done the work for you by listing alternatives for the invasive plants that we feel we cannot live without. View a list of our recommended alternatives to invasive plant species here.

Each of us can play an important role in preserving the natural landscapes for future generations.

The versatility of the willow in the landscape, its year-round ornamental appeal, and its adaptability to colder climates make it a staple and a highlight of the Garden’s plant collections. In fact, the Garden is committed to amassing and displaying one of the largest collections of willows in the country. This initiative was started informally more than two decades ago when Kris Jarantoski, the Garden’s executive vice president and director, began collecting willows from other institutions.

The Garden has 2.4 million plants, and specializes in the cultivation of a select few genera. These specialized collections will become collections of distinction, recognized nationally and internationally.

Willows have long been used by indigenous cultures worldwide. Ancient cultures used the willow as medicine (aspirin derives from salicylic acid—a component found in willow bark); as weaving material for baskets; and for creating shelter. These days, willows are used in furniture; as a material in cricket bats; and as an ornamental landscape plant.

Shrub willows like this Salix tarraconensis are a highlight of the Garden’s specialized collections.

The willow genus (Salix) contains more than 400 species. Derived from the Celtic word sallis—sal ‘near’ and lis ‘water’—their genus name describes the ideal natural habitat of most willows. Despite a natural affinity for water, however, many members of this diverse genus are adaptable to various landscape conditions, including dry sites (once established). Most are native to the cooler regions of the Northern Hemisphere, but a few species occur naturally in the Southern Hemisphere. (Australia does not have native willow species, although willows are cultivated there.)

Developing a world-class collection of willows is a team effort

The process of developing a specialized collection involves much more than acquiring as many difference species and varieties as possible. It also involves ongoing research on these collections by Garden staff and others. But before that is possible, it is essential the collection is authenticated—just as an art museum would do with a painting it received.

Willow twigs are a colorful highlight of the winter landscape. Shown here are twigs from four varieties in the collection.

Willows are a complex and difficult group to accurately identify, and the Garden is currently in the process of verifying its holdings—a process that we believe will take nearly three years!

Emily Russell, assistant curator of woody plants, and Frank Balestri, research assistant at the Garden, work with our collaborators Michael Dodge, a willow enthusiast and owner of Vermont Willow Nursery, and Irina Belyaeva, Ph.D., taxonomist at the Royal Botanic Gardens, Kew.

Russell and Balestri and their teams of photo documentation and herbarium voucher staff and volunteers have devoted countless hours to the willow effort. Russell accompanies Dodge as he surveys the Garden’s collections during visits, and records new information as it becomes available. Balestri’s primary role has been to collect herbarium vouchers prepared by volunteers that will be shipped to Dr. Belyaeva early next year. As we near the end of year two, we are still very busy in these winter months as we continue to collect digital images and herbarium vouchers!

Emily Russell and Michael Dodge look at alpine Salix in the rock garden.

Frank Balestri examines a Salix herbarium voucher

While we continue this project over the coming months, we encourage you to visit the Garden and explore our Salix collection. Winter is a great time to explore the wonderful world of willows!

At the Chicago Botanic Garden, we not only acquire and display our collections, but we must also keep them alive and healthy. As curator of the Garden’s collection of woody plants, I’m responsible for the welfare of more than 13,000 trees. Disease, infestations, and extreme weather events are the kinds of things that keep me awake at night.

As you may know, the Garden is undertaking a ten-year plan to remove about 400 trees due to the emerald ash borer. It’s up to me to suggest suitable replacements to continue our tree legacy. To do so, I first needed to know which of the trees now growing in the Garden would continue to thrive in a warming urban environment. Thanks to a $120,000 research grant from the U.S. Institute of Museum and Library Services, we’ve been able to undertake an adaptive planting study to identify which trees will continue to thrive in Chicago’s urban forests under worst-case carbon-emissions scenarios.

Climate-change modeling indicates that some trees—those currently growing at the northern edge of their hardiness—will actually do a little bit better in slightly warmer conditions around 2020, but by 2050, ten of the 50 trees under study—20 percent—will no longer find the metropolitan area a welcoming habitat. The real concern sets in when we look at the data for 2080, which projects that only 11 of the initial trees would continue to do well in Chicago and the upper Midwest.

The trees growing along our city streets, parks, residences, and public gardens enhance the quality of metropolitan living and also play an important role in reducing greenhouse gas emissions. The cooling summer leaf canopy reduces our energy needs, and the trees themselves store significant amounts of carbon.

Our adaptive planting study suggests two key calls to action: drastically reducing carbon emission to slow climate change and help protect existing trees, and carefully selecting the trees we plant for future generations. To help both public officials and private property owners in their tree selections, the Garden has created an Adaptive Planting page on our website. There you’ll find information on a selection of 60 suitable trees for 2050.